Deposition of insulating films by cathode sputtering in an rf-supported discharge



Nov. 22, 1966 .1.R. LlGr-:NzA 3,287,243

DEPOSITION 0F INSULATING FILMS BY CATHODE SPUTTERING IN AN IiP-SUPPORTED DISCHARGE Filed march 29, 196s o. c. Powfn sup/Lr /N VEN TOR VJ A. L/GE/VZA ATTORNEY DEPOSITION OF INSULATING FILMS BY CAT-Il- VODE SPU'ITERING IN AN RF-SUPPIORTED DIS- CHAR-GE Joseph R. Ligenza, Califon, NJ.,.-assignor to Bell Telephone Laboratories, Incorporated, New York, `NX., a y

corporation of New York Filed Mar. 29, 1965, Ser. No. 446,470 l 9 Claims. (Cl..204-j192) This is a continuation-impart of application, Serial No. 358,473, med April 9, 1964, new abandoned, which films on semiconductor substrates.

The obtaining of a protective film on arfse'miconductor surface finds various commercial uses partifzulafrlyV in the processing of electrical semiconductor devices `such as diodes and ltransistors. In the fabricationof `these def vices oxide films are used for maskingportionsof thev i "also," employed as passivating layers for protecting' tl 1l- Isurface semiconductor to obtain selective diffusion `and" of the device against contamination and leak, Oxide films are employed in maizrygthin-flx'nfl The conventional method for obtaining adeSlKe-doxide layer on a semiconductor body is by growing oxide Patentedv Nov. 22, 1966 A tub diameter. lThe requirements of the microwave field significantly with the specific arrangement of s, in particular, the relative dimensions. urce used with the apparatus described here ci Raytheon Model PGM-IOOCW capable of generating!.300 to 1000 watts of microwave power. The l nd power level can be varied depending upon the subsequent description.

y'the fequlrefnents'of the plasma as specifically defined in The electrode assemblies are constructed of materials which "1:1` ithstand the operating conditions and will not late 4the. system. Thecathode comprises a silicon 2.0.which is sealed into the vclosed end relates to va method for depositing insulatihgor protective i 21 either-.terminal Itube section 22. The tube section 22 is joined. the arm 10 by a 35/25 ball and socket joint 23. The rolf20fis secured within an aluminum sleeve 24 byset screw'jZS'.' The cathode 26 is held in the lower end of thesleve 24 hy set screw 27. The cathode block rchanged to provide the appropriate source ation of the film desired. In particular,

Y silicon, aluminum and beryllium cathode materials provide effectiye. results. i The anode-assembly is assembled. Thisfpermits ready access to the substrate as a result of thermally induced oxidationoffthe lriiicpnductor surface. This practice has long been as inconvenient in the processing of elet:` especially in forming the passivating layer,` cause the electrical characteristics of the othe pleted device suffer harmful effects due to temperatures required to oxidize the semiconductor .slur-v face.

tent ofthe depth of the passivating layer, be reseruedduring processing for subsequent conversion tov -lformtheL oxide. This creates severe problems in fabricating thiniilm devices where thickness tolerances and .diffusion depths are very critical. i

These and other difficulties are in whole or part overcome by the use of the method of this invention. cording Ito the invention an insulating film appropriate ported by an external microwave field. In thismanner oxide films or nitride films can be produced upon the composition of -the plasma gas. l

These and other aspects of the invention will be more easily understood with the aid of the following 'detailed description. In the drawing:

The figure is a front elevation, partly ,in section vand the Y.

partly schematic, of an apparatus appropriateffor practice of this invention.

The apparatus of the figure `consists basically of a Furthermore, the growth of oxide passivating films` dependingv 30. The anode assembly' comprises an anode4 block "31 composed of'aluminum seated into a copper support block '32 by set ,scrap/ 133. A copper sleeve 34 is brazed to support block"32.`1 The sleeve 34 is sealed within Va removable tube section 35. The removable tube section is Odingjoint 'The anode and cathode are connected to a D.C. powersource schematically at 40 whichdelivers 0 tofSOvolts at 0 'to 300 milliamperes. requires that a part of the semiconductor body, to the ern- 231.

quartz tube constructed in four sections or-.arms as The two vertical sections 10 and 11 support the v sired pressure, generally a moderate vacuumoffthe order of a fraction to several millimeters. The tubediameter of the sections 12 and13 is not critical. The tube used in the procedures described here was 1 cm. LD.

The plasma generated in the section 12 is supported by an external microwave field which is coupled to the tube by a tapered waveguide 16. The width 'of the;slot17 of the waveguide corresponds approximately tothe outside i'lhe appara-tus is completed by an RF generator shown schematically at 50 which creates an RF'field 'across'the Esllrface' of the cathode 26. Gold leaf rings Sfahd '542, placed s'indicated, provide the desired field. yThe positicul,Y of the electrodes is not critical as -long as the field is vgenera-terri at the cathodesurface.

An essential aspect ofthe inventionis the useof the medium 'density plasma which provides a region of high,-

V .fly'reactive ions between the source material 26 and the for the uses mentioned above is deposited on thesurt'ace of a semiconductor substrate under specially lsc :lectedconnv ditions with the aidyof a moderate densityplasrna sup? ubs'trate30.' As atoms are sputtered under the influence ,f fthejl'lC potential from the cathode, they combine fgafs species probably at or near the r,cathode-plasma Y and deposit on the substrate surface. 'It has heelal (guild that if the substrate surfacefis itself main'- under a condition of intense high energy bombardough contact with the plasma, the ions lbeing atfthesubstrate surface retain sufiicient'activaf ergyfto migrate to a position of low free energy. 'rrfacermigratiom after deposition, permits the film ode csituniformly without an interfacial discontinuity. t enffouvndA desirable to support the plasma extern g reaction zone by using a microwave "signal dischargi ugha portion of the reactive gas atmosphere.`

p 'conditions describedhere theplasma will form Kalongasnhstantial portion of the tubelfthel'extent 'of which isreadilyevident to the eye. The eXtento'f the plasma ina typical case is indicated in the figure-.VVA 'A arfullu'nderstanding of the invention the nature of the plasma'fand its function in the transferv mechanism will be expl ined. I

A pl i' ,a gas which contains an equal number 'of positive-and negative charges. An ideal plasma iscommonlyl thought of as being composed wholly of'electrons and positively,"`sin`gly charged ions. It is also required that thefgasbe totally ionized.. Plasmas realized in practice are only partially ionized and often contain somt-negative ions."A vlSrome of the positive ions bear more than a Isinglecliarge'and as a rule the positive and negative also made so as to befealsily diss 3 charges are not exactly equal. A plasma generated by microwave, RF or D.C. power is not self-sustaining; energy must be fed into the plasma to maintain it. Energy is lost from a plasma by emission of radiation and by the recombination of positive and negative charges. At low pressures in the range 0.1 to mm. the charge recombination reaction takes place exclusively on any wall available to the plasma. This occurs since a third body must be present to carry `away the energy of the .recombination (three body) reaction and at these pressures, three body collisions are extremely rare. Electrons always have a much larger mobility than does any ion. Consequently, in low pressure discharges, electrons outrace positive ions to any wall and therefore the wall is always negatively charged with respect to the plasma. The plasma does not possess an exactly equal number of positive and negative charges for this reason. AThe greater the concentration of electrons (plasma density) and the greater the electron mobility (electron temperature) the larger the wallpotcntial. The larger the wall potential the greater the energy to which ions will be accelerated to the wall for neutralization. Consequently for a surface toreeeive intense ion bombardment it need not be made a real cath-I ode" with a battery. It can receive the same intense ionic bombardment if it is immersed in a dense plasma where it will become a virtual cathode because of the large wall potential.

However, to extractA metal atoms from the source material it is necessary to impose a D.C. potential between the source cathode and the substrate. This potential.

combined with the wall potential serves to attract `the plasma ions to the cathode surface.` Consequently, to effectively utilize the wall potential to aid `in establishing a saturation density of plasma ions, the cathode is also maintained in direct contact with the plasma. When the apdown region and expands its energy rapidly, locally` heat-` ing the film `and underlying silicon to the melting-pointw and in this manner causes the violent ejection of molten material. These arcs appear, at random, in positions over the entire cathode surface (which is immersed in the gas plasma). The arcs increase in violence and frequency of occurrence as the voltage is advanced above 1 25 volts.

Thus the sputtering of the cathode material into .the`

reactive plasma region for subsequent deposit on Athe substrate as a film does not proceed as might be expected, but is defeated by the ejection of minute masses of the source material. The substrate soon evidences a disorganized ing the cathode potential to bring the cathode to the wall* potential and neutralize the charge layer.

plied potential exceeds approximately 25 volts the cathode region will be saturated with gas ion species. Any voltage above 25 volts will not draw more. than the saturation positive ion current density.

The rate at which atoms will be ejected from the cathode willbe proportional to the product of the sputtering yield times the current density times the area of the cathode.

The sputtering yield increases with increasing voltage. As`

the voltage impressed on the positive ions increases, the depth of penetration of ions into the cathode will also increase. The sputtering yield is the ratio of atoms ejected per ion incidence. For the purposes of this invention this ratio should-never exceed unity The cathode current density should be maintained in the range 25 to 35 milliamperes per cm2.

The selection of the proper D.C. potential is important to the success of the invention. Below 125 volts, ion penetration into the solid is superficial and sputtering is very slight. Above 125 volts sparks will form -all kover the `cathode surface. These sparks are tiny nonself sustaining arcs and are accompanied by momentary large current surges and the ejection of molten globules of silicon. These globules deposit on the substrate and interfere with the deposition. The mechanism for the formation of the arcs is believed to be the following. At the higher voltages the ion penetration is deeper into the solid. Since the sputtering yield is less than unity there will be film formation over the entire cathode surface. This film forms an insulating barrier and when the film becomes thick enough, the plasma ions arriving at the film surface will not be discharged as rapidly as they arrive. Consequently an ion layer will be formed on the cathode. The presence of this positive charge layer docs two things: (1) it will set up a field across the film, and (2) this field will slow down `oncoming positive ions from the plasma and thus reduce the sputtering rate. The magnitude of the field is directly proportional to the surface concentration of the positive ions. When the field exceeds the dielectric breakdown strength of the insulating film a small are will form. The accumulatedvharge rushes into the break- In this manner effective sputtering `potentials in the frange volts to 500 volts can. `.be realized. Such `a periodic interruption in the cathode potenti-ail is most conveniently achieved by means of a `radio frequency;

oscillator. The 'cathode is made a simultaneous RF electrode. Consider the RFvoltage on 'the kcathode alone,

without a simultaneous D.C. voltage. On the nega-tive h-alif cycle the electrode will 'be negative with respect to the plasma at a potential sinusoidally varying with time; plus -the wail potent-iai. positive half cycle the RF electrode can assume only the negative wall potential with respect to the plasma. Thus on lthe negative half cycle positive plasma 4ions (such as 02+ and N2+) ions are accelerated to the cathode surface.

These ions may accumulate on the lcathode surface, but

before they can build up on sufficient density to cause dielectric breakdown, the RF voltage lhas passed to the positive half cycle a-nd the ions become yneuttrzdized Iby the plasma electrons. The applied D.C. `vt'rltage must bc smaller in 'magnitude than the Vpeak RF voltage so that there will always be a part of a cycle where plasma electrous will arrive at the cathode. Thus in this manner one can sputter the `cathode in a dense gas plasma -atany voltage above 125 volts, to obtain very large sputtering rates, without incurring any sparking. The minimum frequency effective for this purpose is approximately 50,000 cycles/sec.

The requirements of the `according lto the principles of this invention can be characterized in terms of its saturation current den-sity for a given gas pressure range. The ygas pressures found to be most useful lie in the range :0.1 mm. to 10 mm. The 1.

saturation current density is a parameter known in the art and described by Review 80, 58 (1950). The preferred range of th'is'parameter is in the range 0.1 ma./em.2 Ito 100 ma./cm.2. If`

the saturation current density falls below this range the deposition pnoceeds very slowly. At saturation current densities in excess of this range the substrate overhcats.

The following examples using the a paratus of the figure illustrate Athe procedure of this invention.

Example I l Silicon oxide (vitreous SI02) was deposited on xa silicon substrate 30. The cathode iblock 26 was composed of As the RF voltage passes to the 1 plasma for effective operation yJohnson and Malter in Physical` and 13 at a `pressure of appr0xi-` The plasma was gener-ated in the density was 0.030 amps/cm?.

approximately 1013 electrons/cm?. The voltage supplied at 40 was 350 volts and the .positive ion current 'Ilhe RF oscillator 50 delivered 500 volts at a frequency of 27 mc. During the deposition the plasma enveloped yboth the anode and cathode. The interelectrode distance was approximately 1.7 cm. The electrode separation is critical for two reasons. Each electrode must Contact the plasma, and the spacing is preferably within 0.5 ern. 'to 5 om. The silicon cathode is sputtered at the equivalent vrate of about 2000 A. of SiOg per minute. Some of the sputtered material is -lost by deposition onto the adjant walls of the tube. The final deposition rate at the anode is about 300 A. of SiOz per minute. At the safrne time the anode is receiving an oxygen ion bombardment of .030 arnps/ cm.2 which corresponds to an ion arrival rate at `the electrode sul-.face of 2-10 ions/ cm3/sec. and since the average surface density of a solid is of the order of 1015 atoms/cm?, these operating conditions result in an ion bombardment of 200 monoalayers of` ions per second. The 300 A./min. SiO, deposition rate corresponds to 1.5 monolayers of SiOz/sec. ilm having a uniform thickness of 10,000 A. and exceptional quality was deposited in the substrate. Substrate materials other than silicon, such as germanium, tantalurn and aluminum are equally elective. 'The sole requirement of the substrate is that it be stable and nonvolatile at the reaction conditions.

Example Il In this example a vitreous silica In this example, nitride films were produced.

Nitride lilms are especially attractive in certain cases where the substrate is more stable n the nitrogen plasma than in a similarly formed oxygen plasma. This is' peculiar of germanium, cadmium sulfide and `gallium arsenide substrates, for instance.

The operating conditions were the following:

Atmosphere Z50-300 microns of nitrogen. Microwave power 300 watts.

RF power 2000 volts; 200 ma.

D.C. power 320 volts; 95 ma.

Substrate temperature-.. 25 C. to 300 C.

Deposition rate 175 A./mfi-.n.

The silicon nitride films were deposited on silicon substrates and germanium substrates. Si3N4 tilms deposited on germanium substrates could ybe stripped from the substrate. These films were examined by electron dilraction and a-Si3N4 was identified.

For the passivation experiments, three diode wafers were coated with 6000 A. of silin nitride; all three exhibited ntype inversion-layer characteristics and conductivities (charge density Q=2Xl011 chgs./cm.2). Upon removal of the films no bulk damage to the was observed.

This film also masks against phosphorus diffusions and it presents no etching or photoresist problems. It is a hard dense film and is dissolved -by dilute hydrouoric acid solutions.

Example III The same conditions described in Example II were used to deposit silicon nitride tlms onto gallium arsenide and cadmium sulde substrates. Since gailium arsenide tends to form a troublesome vapor, care must -be taken to avoid excessive heating of the substrate from any excess microwave energy present. This simply requires that the waveguide be placed so as not to couple too closely with the region adjacent the gal-lium arsenide substrate. The tilms obtained were similar to those of Example II. Other substrate materia-ls such as -tantalum, quartz and glasscanbeusedinasimi'larmanner.

. 6 Example IV Using an aluminum cathode source (99.9999% purity) aluminum nitride lilms, having wurtzite structures, were deposited on gallium arsenide substrates.

Example V To illustrate Ithe versatility of the procedure of this invention, furtherruns were made using different materials. The operating conditions were the same as those described in Example I. Using cathodes com-posed of beryllium and aluminum, high quality beryllium oxide, `beryllium nitride a-nd aluminum oxide lfilms can be formed on substrates suoh as fgold, aluminum, tantalum, silicon, germalnium, III-V semiconductor materials such as GaAs, Gal), and 1I-VI semiconductors such as CdS and on quartz, BeO and glass. In every case the films can be formed at 'temperatures well fbelcw :the melting point of the substrate materia-l. Cambide lms are formed in cases where the plasma Igas is methane. The tact that the basic procedure of the invention can be applied directly -to a variety of materials illustrates the unusual versatility of 4the process.

Various other modilications and extensions of this invention will become apparent to those skilled in the art. All such variations and deviations which basically rely on the teachings through which this invention has advanced theart are properly considered within the spirit and scope of this invention What is claimed is:

1. A method for depositing an insulating tlm on a Isolid substrate which comprises the steps of contacting an anode substrate and ya cathode 'body composed of a material selected yfrom the group consisting of silicon, beryllium and aluminum, said anode and cathode spaced at a distance of 0.5 cm. to 5 cm. with a moderate density gas plasma, said plasma characterized by e gas pressure of 0.1 mm. to 10 mm. said gas selected fromthe group consisting of oxygen, nitrogen and methane, 4and further characterized by a positive ion saturation current density in the range 0.1 [to ma./cm.2, impressing a D C. voltage of at least volts between said anode and cathode and applying an RF signal with a frequency i-n excess of 50 kc. and a voltage in excess of said D.C. voltage to the interface between the cathode and the plasma so as to permit continued uniform deposition of Ithe insulating film on the substrate.

2. The method of cl-aim 1 wherein the -gas is oxygen.

3. The method of claim 1 wherein the gas is nitrogen.

4. The method of claim 1 wherein the gas is methane.

5. The method of claim 1 in which the anode substrate is a material which is stable and nonvolatile under the reaction conditions.

6. The method of claim 1 in which the auode substr-ate comprises silicon.

7. The method of claim 3 wherein the substrate is gallium arsenide.

8. The method of claim 3 manium.

9. The method of claim 3 wherein the substrate is cadmium sulfide.

wherein the substrate is ger- References Cited by the Examiner UNITED STATES PATENTS 3,021,271 2/ 1962 Wehner 204-192 3,108,900 10/1963 Papp 1l7-93.1

JOHN H. MACK, Primary Examiner.

IR. MIHALEK, Assistant Examiner. 

1. A METHOD FOR DEPOSITINNG AN INSULATING FILM ON A SOLID SUBSTRATE WHICH COMPRISES THE STEPS OF CONTACTING AN ANODE SUBSTRATE AND A CATHODE BODY COMPOSED OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON, BERYLLIUM AND ALUMINNUM, SAID ANODE AND CATHODE SPACED AT A DISTANCE OF 0.5 CM. TO 5 CM. WITH A MODERATE DENSITY GAS PLASMA, SAID PLASMA CHARACTERIZED BY A GAS PRESSURE OF 0.1 MM. TO 10MM. SAID GAS SELECTED FROM THE GROUP CONSISTING OF OXYGEN, NITROGENN AND METHANE, AND FURTHER CHARACTERIZED BY A POSITIVE IONN SATURATION CURRENT DENSITY IN THE RANGE 0.1 TO 100 ,A./CM.2, IMPRESSING A D.C. VOLTAGE OF AT LEAST 125 VOLTS BETWEEN SAID ANNODE AND CATHODE AND APPLYING AN RF SIGNAL WITH A FREQUENCY IN EXCESS OF 50 KC. AND A VOLTAGE IN EXCESS OF SAID D.C. VOLTAGE TO THE INTERFACE BETWEEN THE CATHODE AND THE PLASMA SO AS TO PERMIT CONTINUED UNIFORM DEPOSITION OF THE INSULATING FILM ON THE SUBSTRATE. 